OBD key is a bluetooth device, $180 delivered from obdkey.com
I took the picture with a program called Screenshot
http://www.antonypranata.com/screenshot
Free program!
OBD reader is a bluetooth device. SO IT IS WIRELESS and draw the power from the OBD port. There is program for Windows CE smart phone and Palm or Windows CE PDAs. So you can use it as a gauge. But it does not come with a display unit. It is faster than CarBus I was told because it is capable of 4Hz update. 4Hz means 4 signals per second. So if you are requiring information from 4 parameters (speed, rpm, intake temp and ambient temp e.g.), you only have one update per second. Certainly it is not as fast as solid wire!
Speedometer calculate the speed from the drive chain, mostly at the differential level so that it will not be affect by gearing. But if the wheel is changed size (e.g. to different proflie, hence wheel circumference)... the speedo will be affected.
GPS is calculated from several coordinates divided by the time travelled. In a way it will be more accurate since most GPS receiver now log on to 6-8 satellites to get the coordinate. The chances of getting accurate result is very high. Most people trust the GPS speed rather than speedo...
The fact that Kluger's dash speed is different from the OBD/ECU speed is that perplex me... :( . Some cars like to display digital speedo and they are directly from the ECU ... even the Echo/Yaris has one!
Nerdygirl, if you want what good speed it is from the GPS, borrow your friends hand held navigator! Most of them show speed and you can use that to mark out what you want to do above! ;) You don't need a OBD gauge... if you really really want one. Check this out http://www.plxdevices.com/